(1) Field of the Invention
The present invention relates to a conical screw blender.
(2) Description of the Related Art
In the production of many foods, beverages, and nutritional or pharmaceutical products, intermediate or final products may require substantial mixing and/or drying. It is important in these industries that the mixing and/or drying be accomplished efficiently and thoroughly without unnecessarily damaging the materials being mixed or dried. For materials that are sensitive to elements such as heat or moisture, these considerations may be particularly relevant.
For example, probiotics, which are defined as living microbial cell preparations or components of microbial cells that have a beneficial effect on the health and well being of a host, are often added to food or beverage products because they have various health benefits for consumers. Salminen S., et al., Probiotics: How Should They Be Defined? Trend Food Sci. Technol. 10:107-10 (1999). These health benefits may include inhibition of bacterial pathogens, reduction of colon cancer risk, stimulation of immune response, and reduction of serum cholesterol levels. In many situations, probiotics are incorporated into nutritional supplements, children's enteral products, and infant formulas.
Probiotics are adversely affected by four elements: light, heat, oxygen and moisture. Probiotic stability is achieved by minimizing these four elements during production and storage, including during the time that the probiotics are being incorporated into the nutritional or food products. Thus, mixing and drying methods for materials containing probiotics or other similar materials should attempt to eliminate light, heat, oxygen, and moisture.
Generally speaking, the mixing of materials can be accomplished in many ways. As examples, a continuous mixer or a batch mixer may be utilized. A continuous mixer is a process line vessel that is continuously fed the correct proportion of ingredients. The ingredients are quickly mixed, agitated, and discharged to the next piece of equipment in the process. A batch mixer is a stand-alone vessel in which all the ingredients are loaded, agitated until homogeneously dispersed or mixed, and then discharged. A batch mixer is well-suited to applications requiring high mixing accuracy and validation of batch-to-batch consistency. Hixon & Ruschmann, Using a Conical Screw Mixer for More than Mixing, Powder and Bulk Engr. 37-43 (January 1992).
One of the most versatile batch mixers is a conical screw mixer, also referred to as a vertical orbiting screw mixer or conical screw blender. These mixers can handle exclusively dry ingredients, such as powders, as well as combinations of dry and liquid ingredients, such as slurries or pastes. A conical screw mixer can be designed to handle large quantities of material while permitting accurate ingredient and additive proportioning.
A typical conical screw mixer has a vessel which is shaped like an inverted cone. A material inlet is typically located near the top of the vessel and a material outlet is typically located near the bottom of the vessel. A drive motor is often mounted on the top of the vessel and is linked to an orbital arm inside the vessel's top. A cantilevered screw is mounted onto the orbital arm. The cantilevered screw allows near-complete discharge of the vessel contents after mixing. The screw can also be supported at the bottom of the vessel for large batches or viscous materials.
In operation, the drive motor moves the orbiting arm and, in turn, the screw around the vessel's inner wall. As the screw orbits the vessel, the screw also rotates, directing material upward.
The various movements of the materials inside the conical screw blender each contribute to its mixing effectiveness. Firstly, the screw revolves around the blender's own axis and pushes the material upward. Secondly, the orbiting arm causes the materials to be mixed in a circular motion. Lastly, the material that has been pushed upward by the screw descends slowly through the center of the vessel, mixing with the materials being moved upward by the orbiting screw. These various motions ensure thorough mixing, both vertically as well as horizontally.
While there are certain advantages to using conical screw blenders, there disadvantages as well. One major disadvantage in the use of conical screw blenders is the effect of shear force on the materials contained therein. Shear force is a force that acts parallel to a surface. When shear force acts over a certain area, it is referred to as shear stress. In a conical screw blender, gravity pulls the materials downward, creating shear stress due to the compaction of the materials. Shear stress can detrimentally damage materials within a blender, especially if any viable microorganisms are being mixed.
In addition to the shear stress created in a conical screw blender, the mixing process also generates a great deal of friction. This increased level of friction increases the temperature of the materials being mixed. This can also be detrimental to heat-sensitive materials such as probiotics.
In addition to mixing, conical blenders can also serve as drying containers for particulate and biological substrates. Several adaptations such as vacuum pumps or hot-air inlets can be added to a conical mixer in order to make it function as a drying apparatus. Vacuum pumps, however, are inherently expensive to produce, operate, and maintain and it is often difficult to control the product temperature in a vacuum dryer. Vacuum dryers also require a long time period to bring material to high degree of dryness and, thus, have not made conventional conical blenders completely acceptable as drying apparatus. Similarly disadvantageous, hot air-driven conical mixers can be detrimental to heat-sensitive materials, such as probiotics.
The prior art does not provide a conical screw blender that effectively reduces shear force and avoids the generation of friction and heat in the materials being blended. Accordingly, it would be useful to provide a conical screw blender that is useful in drying materials, while at the same time reducing shear stress and friction within the vessel.